1. Field of Invention
This invention is directed to external cavity semiconductor lasers.
2. Description of Related Art
Tunable external cavity diode lasers (ECDLs) are widely used in light-based test and measurement equipment and are increasingly used in wavelength division multiplexed (WDM) optical voice and data communications devices. In some tunable external cavity diode lasers the output is tuned by systems that include an interference filter inserted into the cavity, or by a grating element inserted into the cavity.
Multiple longitudinal modes may be supported by a cavity, corresponding to wavelengths such that an integral number of half-wavelengths can fit within the cavity length. If the gain medium provides gain at the wavelength of the adjacent modes, mode hopping can occur, when, while changing the tuning of the external cavity, an adjacent spectral mode begins to oscillate. The cavity may support the multiple modes, or may “hop” between the modes, by outputting a discontinuous signal on multiple wavelengths.
Recently, various compact systems have been described that provide continuous mode-hop-free tuning for external cavity semiconductor lasers. Examples of these systems include U.S. Pat. Nos. 6,282,215 and 6,526,071 to Zorabedian, and U.S. Pat. No. 6,115,401 to Scobey, each of which is incorporated herein by reference in its entirety.
The '215 patent discloses a system that uses an interference filter to tune the external cavity laser (ECL) with an absence of mode hopping and reduced feedback from interference and reflections. The '215 patent also discloses a tuning mechanism that provides for mechanical frequency modulated tuning of the external cavity laser tuning elements. As disclosed in the '215 patent, a tuning etalon is combined with a wedge-shaped corrective element to provide mode-hop-free tuning. In particular, the tuning element is translated based on an encoder and reader. The required range of motion of the translator along the translation axis may be in the centimeter range in practice. Motor technologies including thermomechanical, ultrasonic, and linear stepper are disclosed. In one embodiment, a voice coil actuator is coupled to and positions the tuning element.
The '071 patent discloses similar tunable laser systems, with a grid generator which exhibits first pass bands which correspond to the individual channels of a selected wavelength grid and a finesse which suppresses side modes of the laser. The external cavity laser described in the '071 patent does not require closed loop feedback. Rather, a tuning mechanism is disclosed which provides for electrical or mechanical tuning to a known position or electrical parameter, e.g., voltage, current, or capacitance. The channel selector exhibits second pass bands that are wider than the first pass bands. The second pass bands have a periodicity that at least substantially corresponds to the separation between the shortest wavelength channel and the longest wavelength channel of the selected wavelength grid and a finesse that suppresses channels adjacent to the selected channel.
The actuators and control systems and methods disclosed in the '215 and '071 patents may be too slow, costly, complex and/or unreliable for various applications.
The '401 patent discloses a system that includes a monolithic prism assembly where the prism assembly comprises a substrate having a first external face and a second external face that opposes the first face and that is inclined at an acute angle to the first face and to a transverse plane of the external cavity. One face of the substrate, preferably the inclined face, incorporates a thin film Fabry-Perot interference filter. Translating the monolithic prism assembly transversely to the optical axis produces continuous mode-hop-free tuning of the laser output wavelength. However, the required properties of the disclosed interference filters may require fabrication that is too precise and/or expensive for various applications. In addition, the required external cavity length that is compatible with practically achievable filters may be too short for including various desirable and economic optical components. Finally, similarly to the '215 and '071 patents, the actuators and control systems and methods disclosed and/or suggested in the '401 patent may be too slow, costly, complex and/or unreliable for various applications.